US7193826B2 - Motor disconnect arrangement for a variable speed drive - Google Patents

Abstract

A motor disconnect arrangement is provided for a variable speed drive having a plurality of inverters electrically connected in parallel to a DC link to power a plurality of motors. The disconnect arrangement includes a contactor or other disconnect device connected in series between each inverter output of the variable speed drive and its corresponding motor. The contactor operates to disconnect, isolate or remove the motor from the variable speed drive in the event of a failure of the motor. By removing a failed motor from the variable speed drive, the other motors connected to the variable speed drive can continue to operate normally.

Description

BACKGROUND OF THE INVENTION

The present invention relates generally to variable speed drives. More specifically, the present invention relates to an arrangement for disconnecting or removing a failed motor from a variable speed drive that has multiple independent inverter outputs.

Many water chiller or refrigeration applications use multiple refrigeration circuits, i.e., two or more refrigeration circuits, each having one or more compressors dedicated to the refrigeration circuit. One purpose of the multiple or redundant refrigerant circuits and compressors is to provide improved reliability of the overall system by having one or more refrigerant circuits and compressors remain operational to provide a reduced level of cooling capacity in the event that a refrigerant circuit and/or compressor fails and can no longer provide cooling capacity.

The corresponding compressor motor for each compressor of a refrigeration circuit can be connected to the AC power grid at the system location. The connection of each compressor motor to the power grid permits the remaining refrigerant circuits and compressors to remain operational even if one refrigerant circuit and/or compressor has a failure. A drawback to connecting the compressor motors to the power grid is that all of the motors are provided only one input voltage and frequency, and thus, can generate only one output speed.

To operate a motor at more than one output speed, a variable speed drive can be inserted between the system power grid and the motor to provide the motor with power at a variable frequency and variable voltage. In the multiple circuit refrigeration system, variable speed operation of the motors can be obtained by providing a corresponding variable speed drive for each compressor motor or by connecting all of the compressor motors in parallel to the inverter output of a variable speed drive. One drawback of using a variable speed drive for each compressor is that the overall chiller system becomes more expensive because multiple drives with a given cumulative power rating are more expensive than a single drive of the same output power rating. One drawback to connecting the compressor motors in parallel to the single inverter output of the variable speed drive is that a fault or failure of one of the motors may disable the variable speed drive and thus prevent the other motors connected to the variable speed drive from operating the remaining compressors on the chiller system. This disabling of the other motors connected to the variable speed drive defeats the function of the redundant refrigerant circuits because all of the refrigerant circuits are disabled as a result of the disabling of the motors and the variable speed drive.

Therefore, what is needed is a disconnect arrangement that can remove or isolate a failed motor connected to a variable speed drive from the other motors connected to the variable speed drive.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a drive system for a plurality of motors having a variable speed drive and a plurality of connecting mechanisms connected in series with the variable speed drive. The variable speed drive includes a converter stage to convert an AC voltage to a DC voltage, a DC link stage to filter and store energy from the converter stage, and an inverter stage having a plurality of inverters electrically connected in parallel to the DC link stage. The converter stage is configured to be electrically connectable to an AC power source. The DC link stage is electrically connected to the converter stage. Each inverter of the plurality of inverters is configured to convert a DC voltage to an AC voltage to power a corresponding motor of the plurality of motors and operates substantially independently of other inverters of the plurality of inverters. Each connecting mechanism of the plurality of connecting mechanisms is connected in series between an inverter of the plurality of inverters and a corresponding motor of the plurality of motors. Each connecting mechanism is configured to disconnect the inverter from the corresponding motor in response to receiving a control signal.

Another embodiment of the present invention is directed to a chiller system having a plurality of compressors incorporated into at least one refrigerant circuit. Each refrigerant circuit has at least one compressor, a condenser arrangement and an evaporator arrangement connected in a closed refrigerant loop. A corresponding motor drives each compressor of the plurality of compressors. A variable speed drive powers the corresponding motors of the plurality of compressors and includes a converter stage, a DC link stage and an inverter stage. The inverter stage has a plurality of inverters each electrically connected in parallel to the DC link stage and each powering a corresponding motor of a compressor of the plurality of compressors. The chiller system also includes a plurality of contactors. Each contactor of the plurality of contactors is connected in series between an inverter of the plurality of inverters and a corresponding motor of a compressor of the plurality of compressors and is configured to enable or disable a connection between the inverter and the corresponding motor of a compressor of the plurality of compressors in response to receiving a control signal.

Still another embodiment of the present invention is directed to a drive system for a multiple compressor chiller system having a plurality of motors. The drive system including a variable speed drive having a converter stage to convert an AC voltage to a DC voltage, a DC link stage to filter and store energy from the converter stage, and an inverter stage having a plurality of inverters electrically connected in parallel to the DC link stage. The converter stage is configured to be electrically connectable to an AC power source. The DC link stage is electrically connected to the converter stage. Each inverter of the plurality of inverters is configured to convert a DC voltage to an AC voltage to power a corresponding motor of the plurality of motors and operates substantially independently of other inverters of the plurality of inverters. The drive system also includes means for isolating a motor of the plurality motor from other motors of the plurality of motors in response to detecting a fault condition in the motor of the plurality of motors.

One advantage of the present invention is that it can isolate a failed motor connected to a variable speed drive without affecting operation of other motors connected to the variable speed drive.

Another advantage of the present invention is that it can be used to reliably and cost-effectively drive multiple motors at variable speeds.

Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a general application of the present invention.

FIG. 2 illustrates schematically a variable speed drive used with the present invention.

FIG. 3 illustrates an embodiment of the present invention used in a refrigeration or chiller system.

Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates generally an application of the present invention. AnAC power source102 supplies a variable speed drive (VSD)104, which powers a plurality ofmotors106. In addition, a plurality ofcontactors108 or other connect/disconnect means or devices are connected in series between theVSD104 and the plurality ofmotors106. Themotors106 are preferably used to drive corresponding compressors of a refrigeration or chiller system (seeFIG. 3).

TheAC power source102 provides single phase or multi-phase (e.g., three phase), fixed voltage, and fixed frequency AC power to theVSD104 from an AC power grid or distribution system that is present at a site. TheAC power source102 preferably can supply an AC voltage or line voltage of 200 V, 230 V, 380 V, 460 V, or 600 V at a line frequency of 50 Hz or 60 Hz to theVSD104, depending on the corresponding AC power grid.

The VSD104 receives AC power having a particular fixed line voltage and fixed line frequency from theAC power source102 and provides AC power to each of themotors106 at desired voltages and desired frequencies, both of which can be varied to satisfy particular requirements. Preferably, theVSD104 can provide AC power to each of themotors106 that may have higher voltages and frequencies and lower voltages and frequencies than the rated voltage and frequency of eachmotor106. In another embodiment, theVSD104 may again provide higher and lower frequencies but only the same or lower voltages than the rated voltage and frequency of eachmotor106.

Acontactor108 is connected in series between each output of theVSD104 and itscorresponding motor106 to disconnect, isolate or remove thatmotor106 from theVSD104 in the event of a failure of themotor106. By removing a failedmotor106 from the VSD104, theother motors106 connected to the VSD104 can continue to operate normally. A microprocessor, controller orcontrol panel110 is used to control thecontactors108 by sending signals (or not sending signals) to thecontactors108 that energize and de-energize thecontactors108 in response to particular system andmotor106 conditions as detected or sensed by sensors, detectors, probes or other similar devices. Thecontactors108 can have normally open contacts that are closed (energized) by thecontrol panel110 during operation of themotors106. If thecontrol panel110 detects a fault, short, ground or other anomalous condition in a motor106 (or possibly the corresponding motor load), thecontrol panel110 can open (de-energize) the contacts in thecontactor108 in order to disconnect or isolate the failedmotor106 from theother motors106 connected to theVSD104. Thecontactor108 can have any suitable arrangement of contacts or other connection devices or mechanisms so long as thecontactor108 can operate to disconnect or isolate a faultedmotor106 from the VSD104. In another embodiment of the present invention, the contacts incontactor108 are normally closed contacts that can be opened (energized) by thecontrol panel110 in response to the detection of a fault in themotor106. By disconnecting or isolating a damaged or failedmotor106 from theVSD104, in particular the DC bus of the DC link of the VSD104 (seeFIG. 2), the VSD104 does not fail and can provide the appropriate power to theremaining motors106 to permit theremaining motors106 to operate normally.

In another embodiment of the present invention, thecontactors108 can be controlled by thecontrol panel110 to enable and disable operation of themotors106 without the detection of a fault or failure in themotor106. The enabling and disabling of themotors106 can be used to control the corresponding motor loads connected to themotors106, e.g., compressors (seeFIG. 3). For example, by disabling amotor106 by de-energizing acontactor108, the corresponding motor load of themotor106 is disabled, which may be desirable depending on the particular application of the motor load. If the motor load is a compressor as discussed above, then the disabling of themotor106 and compressor can be used to adjust the capacity of the system that incorporates themotor106 and compressor. Conversely, the enabling of amotor106 and compressor by energizingcontactor108 can be used to increase the capacity of the corresponding system.

Themotors106 are preferably induction motors that are capable of being operated at variable speeds. The induction motors can have any suitable pole arrangement including two poles, four poles or six poles. However, any suitable motor that can be operated at variable speeds can be used with the present invention.

FIG. 2 illustrates schematically some of the components in one embodiment of theVSD104. TheVSD104 can have three stages: a converter orrectifier stage202, aDC link stage204 and an output stage having a plurality ofinverters206. Theconverter202 converts the fixed line frequency, fixed line voltage AC power from theAC power source102 into DC power. Theconverter202 can be in a rectifier arrangement composed of electronic switches that can only be turned on either by gating, when using silicon controlled rectifiers, or by being forward biased, when using diodes. Alternatively, theconverter202 can be in a converter arrangement composed of electronic switches that can be gated both on and off, to generate a controlled DC voltage and to shape the input current signal to appear sinusoidal, if so desired. The converter arrangement ofconverter202 can have several different configurations including a boost conversion configuration (DC voltage varies from a value equal to the square root of two (2) times the RMS AC input voltage to a value greater than the square root of two (2) times the RMS AC input voltage), a buck conversion configuration (DC voltage varies from zero (0) to a value less than the square root of two (2) times the RMS AC input voltage), and a boost/buck configuration (DC voltage varies from zero (0) to a value that can be greater than or less than the square root of two (2) times the RMS AC input voltage). The converter arrangement ofconverter202 has an additional level of flexibility over the rectifier arrangement, in that the AC power cannot only be rectified to DC power, but that the DC power level can also be controlled to a specific value. The DC link204 filters the DC power from theconverter202 and provides energy storage components such as capacitors and/or inductors. Finally, theinverters206 are connected in parallel on the DC link204 and eachinverter206 converts the DC power from the DC link204 into a variable frequency, variable voltage AC power for acorresponding motor106. The output of eachinverter206 is then connected to a corresponding contactor or connectingmechanism108 which is connected in series between theinverter206 and themotor106.

In a preferred embodiment, theinverters206 are jointly controlled by a control system such that eachinverter206 provides AC power at the same desired voltage and frequency to corresponding motors based on a common control signal or control instruction provided to theinverters206. The use of thecontactors108 to enable and disable themotors106, as discussed above, can be used with the joint control of theinverters206 to provide a further level of control to themotors106. In another embodiment, theinverters206 are individually controlled by a control system to permit eachinverter206 to provide AC power at different desired voltages and frequencies to correspondingmotors106 based on separate control signals or control instructions provided to eachinverter206. This capability permits theinverters206 of theVSD104 to more effectively satisfymotor106 and system demands and loads independent of the requirements ofother motors106 and systems connected toother inverters206. For example, oneinverter206 can be providing full power to amotor106, while anotherinverter206 is providing half power to anothermotor106. The control of theinverters206 in either embodiment can be by thecontrol panel110 or other suitable control device.

For eachmotor106 to be powered by theVSD104, there is acorresponding inverter206 in the output stage of theVSD104 andcontactor108 connected between themotor106 and theinverter206. The number ofmotors106 that can be powered by theVSD104 is dependent upon the number ofinverters206 that are incorporated into theVSD104. In a preferred embodiment, there can be either 2 or 3inverters206 incorporated in theVSD104 that are connected in parallel to the DC link204 and used for powering acorresponding motor106. While it is preferred for theVSD104 to have between 2 and 3inverters206, it is to be understood that more than 3inverters206 can be used so long as the DC link204 can provide and maintain the appropriate DC voltage to each of theinverters206.

In one embodiment of the present invention, theconverter202 can utilize diodes or silicon controlled rectifiers (SCRs) as the power switching mechanisms. The diodes and SCRs can provide theconverter202 with a large current surge capability and a low failure rate. In another embodiment, theconverter202 can utilize a diode or thyristor rectifier coupled to a boost DC/DC converter or a pulse width modulated boost rectifier to provide a boosted DC voltage to the DC link204 in order to obtain an output voltage from theVSD104 greater than the input voltage of theVSD104. The DC link204 can be composed of capacitors and inductors, which are passive devices that exhibit high reliability rates and very low failure rates. Theinverters206 are power modules that can include power transistors or integrated bipolar power transistor (IGBT) power switches with diodes connected in parallel. Furthermore, it is to be understood that theVSD104 can incorporate different components from those discussed above and shown inFIG. 2 so long as theinverters206 of theVSD104 can provide themotors106 with appropriate output voltages and frequencies.

TheVSD104 can prevent large inrush currents from reaching themotors106 during the startup of themotors106. In addition, theinverters206 of theVSD104 can provide theAC power source102 with power having about a unity power factor. Finally, the ability of theVSD104 to adjust both the input voltage and input frequency received by themotor106 permits a system equipped withVSD104 to be operated on a variety of foreign and domestic power grids without having to alter themotors106 for different power sources.

FIG. 3 illustrates generally one embodiment of the present invention incorporated in a refrigeration system. As shown inFIG. 3, the HVAC, refrigeration orliquid chiller system300 has two compressors incorporated in corresponding refrigerant circuits, but it is to be understood that thesystem300 can have one refrigerant circuit or more than two refrigerant circuits for providing the desired system load and more than a single compressor for a corresponding refrigerant circuit. Thesystem300 includes afirst compressor302, asecond compressor303, acondenser arrangement308, expansion devices, a water chiller orevaporator arrangement310 and acontrol panel110. Thecontrol panel110 can include an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board to control operation of therefrigeration system300. Thecontrol panel110 can also be used to control the operation of theVSD104, themotors106, thecontactors108 and thecompressors302 and303. A conventional HVAC, refrigeration orliquid chiller system300 includes many other features that are not shown inFIG. 3. These features have been purposely omitted to simplify the drawing for ease of illustration.

Thecompressors302 and303 compress a refrigerant vapor and deliver it to thecondenser308. Thecompressors302 and303 are preferably connected in separate refrigeration circuits, i.e., the refrigerant output by thecompressors302 and303 are not mixed and travel in separate circuits through thesystem300 before reentering thecompressors302 and303 to begin another cycle. The separate refrigeration circuits preferably use asingle condenser housing308 and asingle evaporator housing310 for the corresponding heat exchanges. Thecondenser housing308 andevaporator housing310 maintain the separate refrigerant circuits either through a partition or other dividing means within the corresponding housing or with separate coil arrangements. In another embodiment of the present invention, the refrigerant output by thecompressors302 and303 can be combined into a single refrigerant circuit to travel through thesystem300 before being separated to reenter thecompressors302 and303.

Thecompressors302 and303 are preferably screw compressors or centrifugal compressors, however the compressors can be any suitable type of compressor including reciprocating compressors, scroll compressors, rotary compressors or other type of compressor. The output capacity of thecompressors302 and303 can be based on the operating speed of thecompressors302 and303, which operating speed is dependent on the output speed of themotors106 driven by theinverters206 of theVSD104. The refrigerant vapor delivered to thecondenser308 enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant fromcondenser308 flows through corresponding expansion devices to anevaporator310.

Theevaporator310 can include connections for a supply line and a return line of a cooling load. A secondary liquid, which is preferably water, but can be any other suitable secondary liquid, e.g., ethylene, calcium chloride brine or sodium chloride brine, travels into theevaporator310 via a return line and exits theevaporator310 via a supply line. The liquid refrigerant in theevaporator310 enters into a heat exchange relationship with the secondary liquid to chill the temperature of the secondary liquid. The refrigerant liquid in theevaporator310 undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in theevaporator310 then returns to thecompressors302 and303 to complete the cycle. It is to be understood that any suitable configuration ofcondenser308 andevaporator310 can be used in thesystem300, provided that the appropriate phase change of the refrigerant in the condenser304 and evaporator306 is obtained.

Preferably, the control panel, microprocessor orcontroller110, in addition to controlling thecontactors108, can provide control signals to theVSD104 to control the operation of theVSD104, and particularly the operation ofinverters206, (and possibly motors106) to provide the optimal operational setting for theVSD104 andmotors106 depending on the particular sensor readings received by thecontrol panel110. For example, in therefrigeration system300 ofFIG. 3, thecontrol panel110 can adjust the output voltage and frequency from theinverters206 to correspond to changing conditions in therefrigeration system300, i.e., thecontrol panel110 can increase or decrease the output voltage and frequency of theinverters206 of theVSD104 in response to increasing or decreasing load conditions on thecompressors302 and303 in order to obtain a desired operating speed of themotors106 and a desired capacity of thecompressors302 and303.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (24)

1. A drive system for a plurality of motors comprising:

a variable speed drive, the variable speed drive comprising:

a converter stage to convert an input AC voltage to a DC voltage, the converter stage being configured to be electrically connectable to an AC power source;

a DC link stage to filter and store energy from the converter stage, the DC link stage being electrically connected to the converter stage;

an inverter stage comprising a plurality of inverters electrically connected in parallel to the DC link stage, each inverter of the plurality of inverters being configured to convert a DC voltage to an output AC voltage to power a corresponding motor of a plurality of motors, and each inverter of the plurality of inverters being configured to operate substantially independently of other inverters of the plurality of inverters; and

wherein the converter stage is configured to provide a boosted DC voltage to the DC link stage and each inverter of the plurality of inverters is configured to provide an output AC voltage greater than the input AC voltage; and

a plurality of connecting mechanisms, each connecting mechanism of the plurality of connecting mechanisms being connected in series between an inverter of the plurality of inverters and a corresponding motor of the plurality of motors, and wherein each connecting mechanism being configured to disconnect an inverter from a corresponding motor in response to receiving a control signal.

2. The drive system ofclaim 1 further comprising a control panel to generate the control signal for a connecting mechanism.

3. The drive system ofclaim 2 wherein the control panel comprises:

means for detecting a fault condition in a motor of the plurality of motors; and

means for generating the control signal for the corresponding connecting mechanism connected to the motor with the detected fault condition in response to the detection of the fault condition in the motor.

4. The drive system ofclaim 2 wherein the control panel comprises means for generating the control signal in response to a control instruction from a control system controlling a corresponding motor load connected to a motor of the plurality of motors.

5. The drive system ofclaim 1 wherein the plurality of connecting mechanisms comprises a plurality of contactors.

6. The drive system ofclaim 5 wherein the plurality of contactors each comprise at least one normally open contact and the control signal de-energizes the at least one normally open contact of a contactor to disconnect an inverter from a corresponding motor.

7. The drive system ofclaim 5 wherein the plurality of contactors each comprise at least one normally closed contact and the control signal energizes the at least one normally closed contact of a contactor to disconnect an inverter from a corresponding motor.

8. A chiller system comprising:

a plurality of compressors, each compressor of the plurality of compressors being driven by a corresponding motor, the plurality of compressors being incorporated into at least one refrigerant circuit, each refrigerant circuit comprising at least one compressor of the plurality of compressors, a condenser arrangement and an evaporator arrangement connected in a closed refrigerant loop;

a variable speed drive to power the corresponding motors of the plurality of compressors, the variable speed drive being configured to provide an output voltage greater than the input voltage to the variable speed drive, the variable speed drive comprising a converter stage, a DC link stage and an inverter stage, the inverter stage having a plurality of inverters each electrically connected in parallel to the DC link stage and each powering a corresponding motor of a compressor of the plurality of compressors;

a plurality of contactors, each contactor of the plurality of contactors being connected in series between an inverter of the plurality of inverters and a corresponding motor of a compressor of the plurality of compressors, and wherein each contactor being configured to enable or disable a connection between the inverter and the corresponding motor of a compressor of the plurality of compressors in response to receiving a control signal.

9. The chiller system ofclaim 8 further comprising a control panel to generate a control signal for each contactor of the plurality of contactors.

10. The chiller system ofclaim 9 wherein the control panel comprises:

means for detecting a fault condition in a corresponding motor of a compressor of the plurality of compressors; and

means for generating a control signal for a corresponding contactor connected to the corresponding motor with the detected fault condition to disable the connection between the inverter and the corresponding motor with the detected fault condition.

11. The chiller system ofclaim 9 wherein the control panel comprises:

means for detecting a fault condition in a corresponding compressor of the plurality of compressors; and

means for generating a control signal for a corresponding contactor connected to a corresponding motor of the compressor of the plurality of compressors with the detected fault condition to disable the connection between the inverter and the corresponding motor of the compressor of the plurality of compressors with the detected fault condition.

12. The chiller system ofclaim 9 wherein the control panel comprises means for generating a control signal for a corresponding contactor connected to a corresponding motor of a compressor of the plurality of compressors to enable the connection between the inverter and the corresponding motor of a compressor of the plurality of compressors.

13. The chiller system ofclaim 8 wherein the plurality of contactors comprise a plurality of normally open contacts.

14. The chiller system ofclaim 13 wherein the control signal de-energizes the normally open contacts to disable the connection between an inverter and a corresponding motor of a compressor of the plurality of compressors.

15. The chiller system ofclaim 13 wherein the control signal energizes the normally open contacts to enable the connection between an inverter and a corresponding motor of a compressor of the plurality of compressors.

16. The chiller system ofclaim 8 wherein the plurality of contactors comprise a plurality of normally closed contacts.

17. The chiller system ofclaim 16 wherein the control signal energizes the normally closed contacts to disable the connection between an inverter and a corresponding motor of a compressor of the plurality of compressors.

18. The chiller system ofclaim 16 wherein the control signal de-energizes the normally closed contacts to enable the connection between an inverter and a corresponding motor of a compressor of the plurality of compressors.

19. A drive system for a multiple compressor chiller system having a plurality of motors, the drive system comprising:

a variable speed drive, the variable speed drive comprising:

a converter stage to convert an input AC voltage to a DC voltage, the converter stage being configured to be electrically connectable to an AC power source;

a DC link stage to filter and store energy from the converter stage, the DC link stage being electrically connected to the converter stage;

an inverter stage comprising a plurality of inverters electrically connected in parallel to the DC link stage, each inverter of the plurality of inverters being configured to convert a DC voltage to an output AC voltage to power a corresponding motor of a plurality of motors, and each inverter of the plurality of inverters being configured to operate substantially independently of other inverters of the plurality of inverters; and

wherein the converter stage is configured to provide a boosted DC voltage to the DC link stage and each inverter of the plurality of inverters is configured to provide an output AC voltage greater than the input AC voltage;and

means for isolating a motor of the plurality of motors from other motors of the plurality of motors in response to detecting a fault condition in the motor of the plurality of motors.

20. The drive system ofclaim 19 wherein the means for isolating a motor comprises a plurality of contactors, each contactor of the plurality of contactors being connected in series between an inverter of the plurality of inverters and a corresponding motor of the plurality of motors, and wherein each contactor being configured to disconnect an inverter from a corresponding motor of the plurality of motors with a detected fault condition.

21. The drive system ofclaim 20 wherein the plurality of contactors comprise a plurality of normally open contacts.

22. The chiller system ofclaim 21 wherein the normally open contacts are de-energized to disconnect an inverter from a corresponding motor with a detected fault condition.

23. The chiller system ofclaim 20 wherein the plurality of contactors comprise a plurality of normally closed contacts.

24. The chiller system ofclaim 23 wherein the normally closed contacts are energized to disconnect an inverter from a corresponding motor of the plurality of motors with a detected fault condition.

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